The invention relates to an arrangement for combining the output signals from a plurality of transmission units tuned to the same frequency and included in the arrangement, and comprises for each pair of transmission unit control circuitry for generating a control signal used to combine the output signals of the two transmission units in phase. If two output signals are combined to form one signal .SIGMA., a first condition for maximum power of the .SIGMA. signal, i.e. the sum of the power levels of the two output signals, is that the pair of output signals to be combined be of the same frequency. A second condition for a maximum power level of the .SIGMA. signal is that the power levels of the two output signals to be combined be equal to one another. A third, final condition for a maximum power level of the .SIGMA. signal is that the reduced phase difference .phi. of the output signals to be combined is 0.degree.. The first condition is fulfilled simply by controlling the two transmission units, whose output signals are to be combined, with a common frequency control signal. The second condition is fulfilled simply by ensuring that the two transmission units, whose output signals are to be combined, are identical to each other. The problem is now confined to the minimization of the reduced phase difference .phi.. An arrangement to minimize .phi. is known from Dutch patent application No. 6614897. The latter arrangement has however the disadvantage that the output signals in question are first detected and corrected for any phase difference .phi. and subsequently combined to form one signal.
Any phase error arising after the detection of a phase error by a detection element, for example due to differences in delay of the output signals to be combined, is no longer detected and corrected. Also phase errors arise due to a poor standing wave ratio at the input of the detection element and due to a DC bias and phase offset in the detection element. In this way, a phase error in the order of 30.degree. may even be incurred without making due correction for such an error.